Electroplating solution

ABSTRACT

A conversion coating solution comprises at least one precursor agent, at least one activator agent, and water solvent, wherein on provision to a solution suitable for the deposition of a substantially metallic coating, provides an in-situ conversion coating to the surface of the substantially metallic coating.

BACKGROUND

[0001] This invention related to a conversion coating solution, aprocess that provides conversion coatings to substantially metalliccoatings during or following electroplating, and articles treated usingthe solution and process.

[0002] Coatings that consist substantially of metallic elements areprovided to substrate materials such as steels on an industrial scale.The most usual method is through electrodeposition from aqueoussolutions, known in the art as electroplating. A further method for thedeposition of substantially metallic coatings is electroless plating,also familiar to those practiced in the art. Metallic coatings are oftennot deposited as a pure metal, due to side reactions such as thedeposition of hydrogen, the uptake of precipitated substances, or theintentional inclusion of elements, and it is more accurate to employ theterm ‘substantially metallic’.

[0003] Substantially metallic coatings are used to obtain propertiesthat cannot be afforded by the substrate material alone. Theseproperties include appearance, corrosion protection, and engineering orphysical properties such as improved wear resistance. Chromium is usedfor decorative effects, and is usually deposited over an undercoat ofcopper or nickel. Others include gold, silver, platinum, nickel, copper,and rhodium. For applications where improved engineering or physicalproperties are required, chromium also finds extensive use, as donickel, tin, lead, and silver. In applications where sacrificialcorrosion resistance is required, zinc, and to a lesser extent, cadmium,are the preferred choices. In the case of zinc, it can beelectrodeposited either alone, or in combination with other metals toform alloy coatings, for example, zinc-cobalt, zinc-iron, andzinc-nickel. In some cases, particles are incorporated into metallicdeposits to improve properties such as wear resistance, known in the artas composite coatings. There is a vast range of applications for all ofthese coatings.

[0004] In particular, the electrodeposition of substantially metalliccoatings takes placed on a global scale. It is known to those practisedin the art that the electrodeposition of substantially metallic coatingsrequires the following: an external circuit, consisting of a source ofelectrical current, means of conveying current to the plating cell, andassociated instruments such as ammeters, and means of regulating thevoltage and current; the negative electrodes or cathodes, which are thearticles to be coated, along with a means of positioning the article inthe plating solution so that contact is made with the current source;the plating solution itself, almost always aqueous, and contained withina tank; and the positive electrodes, the anodes, usually of the metalbeing plated.

[0005] The electrolyte most often comprises an aqueous solutioncontaining ions of the metal to be deposited. Sometimes the solutionalso contains ions of a different metal, for example, when an alloydeposit is desired. On the passage of electrical current through thecell, the metal ions are attracted to the article, where they arereduced to form a coating of that metal. The current flow is terminatedwhen the required thickness of coating has been achieved. The coatedarticle is then most usually rinsed several times in water, to removesurplus solution, brought into contact with a neutralising solution tocounteract remaining traces of the electrolyte, and again rinsed inwater. If the coated article is to be subject to conversion coatingtreatment, it is most often then brought into contact with an acid rinseto activate the surface. This basic sequence of rinses and treatmentsmay be modified according to the nature of the metallic coating.

[0006] The various treatment solutions, including rinse water, are mostoften contained within steel tanks often lined with plastic materials.The electrodeposition of steel strip involves continuously moving thestrip through the various solutions. For small items such as fasteners,jigging is avoided through the use of rotating barrels that containelectrolyte solution. The various treatment solutions must be maintainedat a constant temperature, and heating is usually provided through theuse of electrical elements. Large volumes of clean water are requiredparticularly for the numerous rinse stages. The treatment solutions,particularly those used to deposit the metallic coating, require regularchemical analysis to ensure that the formulation is withinspecification. If the solution is outside specification, the appropriatechemicals are added in the required amounts to the solution, thusreturning the solution to within specification. Other solutions, such asthe acid rinse, gradually become contaminated and must be drained fromthe tank and replenished at regular intervals.

[0007] The multiple treatment tanks also produce ‘drag out’. This refersto small amounts of water or solution that adheres to the articlesurface following removal from the tank. This becomes ‘drag in’ when thearticle is then brought into contact with the next solution in thesequence. This solution is thus contaminated with the ‘drag in’. A greatdeal of effort is expended in minimising the effects of ‘drag out’,which depletes solution, and ‘drag in’, which can lead to seriouscontamination problems. One approach is to use continuous filtration ofthe most important solutions to remove contaminating ions.

[0008] Before the article is subject to the application of coatingtreatments, it is understood by those practiced in the art, thatappropriate cleaning procedures must be carried out. For steel articles,solvent degreasing may be used to remove oils and greases, followed bycontact with solutions that render the article surface in a chemicallyclean condition, such that if a water film were applied to the surface,no break in that water film would be observed.

[0009] Solutions that are suitable for use in coating deposition areknown widely in the art. For electrodeposition, proprietary solutionsare available for the full range of substantially metallic coatings. Forother purposes, the compositions of suitable solutions are widelypublished in the open literature, and are well known by those practicedin the art. The solutions all contain metal ions of the metals that areto be deposited onto the article. For acid solutions these are mostoften salts of the metals that are sufficiently soluble in water. Foralkaline solutions, lower concentrations of the metal salts are added toconcentrated solutions of hydroxide salts such as sodium hydroxide.

[0010] The remaining additions to the electroplating solution whollydepend on the qualities that are desired of the substantially metalliccoating, although some general points can be made. Wetting agents arewidely used to reduce the surface tension of the solution, which has theeffect of minimising the formation of pores in the deposit. A commonexample of a wetting agent is sodium lauryl sulfate. The addition to thesolution of complexing agents is sometimes required. These formcomplexes with the metal ions in order to facilitate the deposition ofthe metal, without which their deposition might not be possible. Theaddition of complexing agents is of particular importance with alkalinesolutions. The throwing power of acid solutions is often improvedthrough the addition of salts that increase conductivity, for example,salts of alkali metals or ammonium compounds. For acid solutions, it isoften necessary to stabilise the pH through the addition of buffers suchas boric acid or acetic acid. For all solutions, the physical form ofthe deposit can be modified or regulated through the addition oflevelling agents, which assist in the formation of uniform deposits, orbrightening agents, which promote the deposition of bright coatings.Other chemical additions may be required to aid in the dissolving ofanodes, and to modify other properties, either of the solution or of thedeposit, depending on the specific case.

[0011] Many substantially metallic coatings consisting substantially ofchromium, nickel, rhodium, platinum and gold, possess inherent tarnishresistance, and can be put straight into the use for which they areintended. For others, the coating deposit formed on the article requiresfurther treatment to impart the range of properties demanded by the enduser. These include zinc, cadmium, silver, tin, and copper, and alloyssuch as zinc-cobalt, zinc-nickel, and zinc-iron. In the case of thosecoatings consisting substantially of zinc, the surfaces are often dulland susceptible to staining, and possess inadequate corrosionresistance. Moreover, their tendency to oxidise in the atmosphererenders them unfit for use, and the formation of loosely adherentcorrosion products prevents the successful application of paints.

[0012] For coatings containing substantial quantities of metals such aszinc, cadmium, silver, tin, and copper, the further treatment consistsof the application of a conversion coating. This is carried out in aseries of treatment tanks separate from those involved inelectrodeposition of the coating onto substrate materials such assteels. A conversion coating is known in the art to consist of a coatingproduced by chemical or electrochemical treatment on a substantiallymetallic surface, which gives a superficial layer containing compoundsof the metals present.

[0013] To those practiced in the art, a conversion coating is known notto be necessarily identical to the films provided to substantiallymetallic coatings known as passivation treatments. Conversion coatingsmay act as a barrier layer, in which case the underlying metallicsurface is not rendered to a passive state. Other conversion coatings,for example those containing appreciable amounts of chromate, however,are known to change the surface of some metallic coatings from an activestate to a more passive state, in which case they are sometimes referredto as passivation films.

[0014] The most widely used conversion coating treatments are thosebased on chromates or phosphates, or combinations of chromates andphosphates. These are widely available as proprietary solutions that areformulated for specific substantially metallic coatings. For thosecoatings containing appreciable amounts of cadmium, copper, and zinc, aswell as other metals, treatments containing chromates are the preferredoption. Phosphates are also widely used, for example, on zinc-ironalloys, where they improve paint adhesion. There are different types ofphosphate treatment available, including iron-phosphates andzinc-phosphates as treatments before painting, and manganese phosphates,which provide additional protection against wear.

[0015] Chromate conversion coatings are most often imparted to thesurface of substantially metallic coatings through immersion or sprayingof aqueous solutions of hexavalent chromate salts. The solutions aremost often strongly acidic and the contact time with the coating isusually no more than 30s. This brief contact is sufficient to polish andsmooth the otherwise dull coating surface to a brilliant and specularfinish. The chromate solutions also remove haze or other surface filmsthat would otherwise interfere with specular reflection.

[0016] In most countries, there are many proprietary chromate treatmentsolutions available. They have been used routinely since the early yearsof the twentieth century. They are formulated to impart characteristiccoloured finishes to articles coated with substantially metallicdeposits, and are often called up in specifications. For maximumcorrosion resistance, the yellow chromates are the preferred option,since these conversion coatings contain the highest concentrations ofchromate. These conversion coatings are used on coated steel articlesfrom fasteners on aircraft, to automobile bodies, through to large steelstructures. Olive drab is used on military equipment. Iridescentchromates are less corrosion resistance but impart a distinctivedecorate finish. There are other chromates available including black,which are often used on automobile parts. Chromate finishes can also bedyed for colour coding purposes. Chromate conversion coatings are alsoapplied onto metallic coatings as a base for paints and other organicfinishes. The chromate conversion coating provides ‘anchor points’ towhich the paint film can adhere; it also retards the spread of corrosionif the paint is scratched or damaged.

[0017] The colour and lustre of chromate conversion coatings varyaccording to the nature of the substantially metallic coating, thecomposition of the treatment solution, method of application, and thetreatment conditions such as temperature, time and acidity. It isessential that the conversion coating treatments impart the requiredcolour and reflective brilliance, lest the treated article be rejected.

[0018] Chromate conversion coatings are formed by a chemical reactionbetween the substantially metallic coating surface and the hexavalentchromium in solution. The metals present in the surface of the metalliccoating are ‘converted’ to their oxides, and the hexavalent chromium isreduced to trivalent chromium. The conditions at the surface become morealkaline and this causes the trivalent chromium to precipitate in theform of a gel, which entraps some of the hexavalent chromium solution,thus generating the conversion coating. Chromate conversion coatings arein essence amorphous gels and are considered to ‘harden’ over time. Thisarises from gradual dehydration of the conversion coating, leading tothe formation of micro cracks. Under normal circumstances, this is notconsidered to be detrimental, although excessive heat can dehydrate thegel and damage corrosion resistance.

[0019] The deposition of substantially metallic coatings followed by theapplication of a conversion coating, has formed established practice formany years. There are two drawbacks inherent with the establishedpractice. Firstly, the separate provision of the conversion coatingrequires the use of numerous rinsing and other treatment stagesfollowing electrodeposition. Secondly, the use of hexavalent chromatesis extremely undesirable because they are known to be carcinogenic andharmful to the environment. The chromate solutions, including thetreatment solution, and rinse water used to remove surplus solution,requires extensive treatment before it can be recovered for subsequentreuse, or disposed of. Those containing phosphates are substantiallyless harmful, however, the treatment is of a similarly multi-stagenature.

[0020] The industry has attempted to replace chromate solutions withless toxic alternatives, but again as a separate conversion coatingtreatment stage, with the associated rinsing and treatment solutions.This approach has fallen down on two counts, firstly, the alternativesolutions do not match the overall performance provided by chromates,and, secondly, they are invariably more expensive than chromates.

[0021] The use of chromates or phosphates as conversion coatingstreatments currently requires the use of many treatment stages, andextensive ancillary equipment such as filtration plant and chemicalanalysis facilities. Separate plant is required for the treatment ofwater contaminated with chromates. Any solid chromate waste must bestored or taken to licensed landfill sites. Others employ ion exchangecolumns to separate out the chromates, allowing the cleaned water to bereused. These multiple treatments were once acceptable when the rawmaterials required to service the many treatment stages were relativelyinexpensive, and the treatment of chromate waste less regulated bygovernments. This picture has changed in most developed countries, andwater, electricity, equipment, and factory space are going up in cost.This is exacerbated by the continued use of chromates, and there iscurrently no answer to the threat posed by national and internationallegislation that is likely to prohibit their use.

BRIEF SUMMARY OF THE INVENTION

[0022] The invention enables electroplating and chemical conversioncoating, currently separate processes with numerous intermediate stages,to be conducted from a single solution. No chromates or other toxicsubstances are employed.

DETAILED DESCRIPTION

[0023] In accordance with a first aspect of the present invention, aconversion coating solution comprises at least one precursor agent, atleast one activator agent, and water solvent.

[0024] Preferably, the conversion coating solution further comprises atleast one modifying agent.

[0025] In accordance with a second aspect of the present invention, aprocess for coating an article, wherein a conversion coating solutioncomprises at least one precursor agent, at least one activator agent,and water solvent, wherein on provision to a solution suitable for thedeposition of substantially metallic coatings, imparts a conversioncoating layer to the substantially metallic coating.

[0026] In accordance with a third aspect of the present invention, aprocess for coating an article wherein a conversion coating solutioncomprises at least one precursor agent, and water solvent, wherein useis provided to an aqueous solution suitable for the deposition ofsubstantially metallic coatings, wherein an article is brought intocontact with the solution, wherein use a substantially metallic coatingis formed on the article, the at least one activator agent is providedto the solution, such that a conversion coating layer is imparted to thesubstantially metallic coating.

[0027] In accordance with a fourth aspect of the present invention, aprocess for coating an article wherein a conversion coating solutioncomprises at least one precursor agent, and water solvent, wherein useis provided to an aqueous solution suitable for the electrodeposition ofsubstantially metallic coatings, an article is brought into contact withthe solution, electrical current is passed to the article such that asubstantially metallic coating is deposited onto the article, thecurrent is substantially reduced or halted, the at least one activatoragent is provided to the solution, such that a conversion coating layeris imparted to the substantially metallic coating.

[0028] In accordance with a fifth aspect of the present invention, aprocess for coating an article wherein a conversion coating solutioncomprises at least one activator agent, and water solvent, wherein useis provided to an aqueous solution suitable for the electrodeposition ofsubstantially metallic coatings, an article is brought into contact withthe solution, electrical current is passed to the article such that asubstantially metallic coating is deposited onto the article, thecurrent is substantially reduced or halted, the at least one precursoragent is provided to the solution, such that a conversion coating layeris imparted to the substantially metallic coating.

[0029] The solution suitable for the deposition of a substantiallymetallic coating is employed to provide the metallic coating on anarticle, to the surface of which can be provided the conversion coating.

[0030] To those skilled in the art, a solution suitable for thedeposition of a substantially metallic coating will be recognised as anythat wherein use provides a substantially metallic coating to anarticle. These solutions are widely available either as laboratoryformulations or on a proprietary basis from any supply house.

[0031] Preferably, the solution suitable for the deposition ofsubstantially metallic coatings is an electroplating solution.

[0032] Preferably, the electroplating solution is suitable for thedeposition of substantially metallic coatings comprising the followingmetals: Zn, Fe, Ni, Co, Ag, Sn, Cu, Mn.

[0033] Preferably, the electroplating solution is suitable for thedeposition of substantially metallic coatings comprising the followingmetals: Zn, Fe, Ni, Co.

[0034] Preferably the electroplating solution is acid, alkaline, orsubstantially neutral. Particularly suitable are acidic solutionscontaining counter ions of sulphates, chlorides, or combinations ofsulphates and chlorides.

[0035] The provision of the conversion coating is mostly independentfrom the provision of the metallic coating. The requirement is that theconversion coating solution and process of the present invention doesnot act to interfere with the deposition of the substantially metalliccoating.

[0036] The conversion coating solution comprises at least two parts, theat least one precursor agent and the at least one activator agent.

[0037] The at least one precursor agent is a chemical species that oncontact with the at least one activator agent, reacts to form a solidprecipitate.

[0038] Preferably, the at least one precursor agent is a non-reduciblemetal ion. Preferably it is chosen from a salt of the following: Y³⁺,La³⁺, Ce³⁺, Ce⁴⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺,Tm³⁺, Yb³⁺, Lu³⁺. Suitable salts are chlorides or sulphates, or mixturesof the two.

[0039] More preferably, the at least one precursor agent is selectedfrom Y³⁺, La³⁺, Ce³⁺, Ce⁴⁺, Pr³⁺, Nd³⁺.

[0040] Preferably, the at least one activator agent is peroxide (O₂ ²⁻),peroxonium salts (H2OOH)⁺, hydroperoxide (OOH)⁻, or ozone analogues ofperoxide (HOOOH), or nitrate salts.

[0041] More preferably, the at least one activator agent is hydrogenperoxide.

[0042] The modifying agent is a chemical species that is entrappedduring the formation of the solid precipitate, arising from the reactionbetween the precursor and activator agents. This serves to impartadditional properties on the conversion coating, for example, thoserelating to appearance such as colour or reflection, and properties suchas corrosion resistance.

[0043] Preferably, the at least one modifying agent is an anion ofnon-metals selected from: silicate, oxalate, iodate, formate, phosphate,or carbonate.

[0044] More preferably the at least one modifying agent is silicate orphosphate.

[0045] Preferably, the at least one precursor agent, the at least oneactivator agent, and the at least one modifying agent are dissolved in asolvent prior to being provided to the plating solution.

[0046] Preferably, the solvent used to dissolve the at least oneprecursor agent, the at least one activator agent, and the at least onemodifying agent is water.

[0047] Preferably, the concentration of the precursor agent is in therange 0.001 g/L to saturation.

[0048] Preferably, the concentration of the activator agent is in therange 0.001 g/L to saturation.

[0049] The concentration of hydrogen peroxide requires particularlycareful control lest the rate of conversion coating formation isexcessive, interfering with the initiation and growth of the metalliccoating, and causing the growth of macroscopic dendrites.

[0050] Preferably, the concentration of the modifying agent is in therange 0.001 g/L to saturation.

[0051] Preferably, the precursor, activator, and modifying agents areprovided separately to the solution suitable for the formation ofmetallic deposits.

[0052] More preferably, the precursor agent and modifying agents areprovided to the solution prior to the provision of the activator agent.

[0053] The current invention allows the provision of conversion coatingsfrom the same solution that provides the metallic coating onto anarticle. This substantially reduces the number of treatment stagesrequired in order to provide a conversion coating to a metallic coating.This brings about major advantages over existing practices, includingreduced water consumption, reduced energy consumption, and the need formuch fewer treatment tanks. All of the costs and risks currentlyassociated with chromates are eliminated.

[0054] The plating solution once provided with the at least oneprecursor agent, the at least one activator agent, and the at least onemodifying agent, can be re-used until the aforementioned additions areexhausted, after which they may be replenished so that theirconcentrations are sufficient to permit conversion coating formation.

[0055] The present invention provides conversion coatings that arecolourless, iridescent, or coloured. The formulations and processes canbe chosen to provide conversion coatings that are identical inappearance as those provided by chromates. The process of the presentinvention may be used to treat any article substrate that can be treatedusing traditional chromate or phosphate conversion coating processes.

[0056] For the provision of coloured conversion coatings that areidentical in appearance to those provided by chromates, purely forillustrative purposes, the precursor agent might comprise Ce³⁺, theactivator agent, H₂O₂. The precursor agent is provided to a zinc sulfateplating bath, and the article brought into contact with the bath.Electrical current is passed through the solution such that zinc coatingis provided to the article, and the current substantially reduced orswitched off. The peroxide is provided to the solution. This reacts withCe³⁺ to form a sparingly soluble precipitate of hydrated cerium oxide onthe surface of the metallic deposit, which is gelatinous in substanceand yellow in colour. The precipitate forms a continuous coating layerof approximately 2 μm in thickness that is visually indistinguishablefrom those provided by chromates.

[0057] The activator agent may be provided to the solution before,during, or after, the formation of the substantially metallic coating.

[0058] The precursor agent, the activator agent, and modifying agent,require periodic replenishment. Where the activator constitutesperoxide, this substance decomposes naturally, and must be replenishedfrequently.

[0059] Preferably, the electrical current to reduce the metal ions inthe plating bath, thereby providing a substantially metallic coating tothe article, is direct current. Also suitable is pulsed current, orcurrent with AC superimposed.

[0060] Preferably, the conversion coating once imparted is rinsed incold water to remove surplus solution.

[0061] Preferably, the rinsed conversion coating is further rinsed inwater in the temperature range 20-80° C., but more preferably in thetemperature range 30-55° C.

[0062] Preferably, the hot rinse solution contains the at least oneprecursor agent used in the prior provision of the conversion coating,in the concentration range 0.001g/l to saturation.

[0063] Preferably, to the rinsed conversion coating is imparted asealing agent.

[0064] Preferably, the sealing agent is an organic substance.

[0065] More preferably, the sealing agent is a silane. Suitable silanesare those of the general formula R—Si(OX)₃, where R is a functionalgroup and X is an alkoxy group or other organo-functional group, whichare hydrolysed to silanols of the general formula R—Si(OH)₃. Othersuitable sealing agents are metallo-siloxanes, silicates, silicateesters, polyesters, polyoxyethylenes, titanates, thioglycollates,trithioglycollates, and acrylics.

[0066] Preferably, the article material is a metal such as thoseconsisting substantial quantities of iron, titanium, or aluminium.

[0067] Preferably, the article material is non-metallic, such as aplastic or carbon fibre, onto which a thin substantially metalliccoating has been added.

[0068] More preferably, the article material is steel.

[0069] The invention may be described by way of example only withreference to the following drawings.

[0070]FIG. 1 shows a schematic diagram of a current state of the artconventional electroplating process.

[0071]FIG. 2 shows a schematic diagram of a surface treatment processaccording to the present invention.

[0072]FIG. 3 shows a schematic diagram of a plating cell suitable forthe provision of the conversion coating solution of the presentinvention.

[0073]FIG. 4 shows a profile of the operation of a surface treatmentprocess according to one aspect of the present invention.

[0074]FIG. 1 shows a conventional electroplating process. Each boxrepresents a separate treatment stage, through which the cleaned articleis moved in a sequential manner. The electrodeposition stage providesthe substantially metallic coating to the article. Two rinsing stagesremove surplus solution, remaining solution is neutralised, andneutralising solution removed by water rinsing. An acid rinse removessurface films. The coated article is immersed in a bath to provide thechromate conversion coating. Two rinsing stages remove surplus solution.The treated article is rinsed in hot water and then dried in warm air.The treatment cycle is completed and the treated article is inspectedand stored in readiness for dispatch.

[0075]FIG. 1 shows the flow of clean water to the rinse stages for theremoval of surplus solution following electrodeposition, annotated (1)on the diagram. The water becomes contaminated with metals from theelectrodeposition stage through the effects of drag-in. The rinse waterruns into a general waste stream (2). The water may be sent to atreatment plant, where it is cleaned or disposed. For disposal, thewater may be piped to open tanks where the water is allowed toevaporate. The preferred option is to recover the water through removingcontamination, using techniques such as reverse osmosis or ion exchange.It is then suitable for reuse and returned to the clean water stream(3).

[0076]FIG. 1 shows the flow of clean water to the rinse stages for theremoval of surplus solution following chromate treatment (4). Once againthe water becomes contaminated but this time with hexavalent chromate,and is run into a separate waste stream (5). This requires specialtreatment due to the extreme toxicity of chromate. One approach is torun the water into evaporation tanks, reduce the hexavalent chromate totrivalent chromium through chemical additions, which then precipitatesout. The precipitate is allowed to settle in the tanks to form sludge.The sludge is taken to landfill whilst the water is then treated furtherto remove all traces of chromate, using techniques such as ion exchange.The treated water is then returned to the water stream (6).

[0077]FIG. 2 illustrates a process of the present invention to allowcomparison with traditional electroplating practices as outlined above.The conversion coating solutions are held in a series of automatictitrators (7) that provide the solutions to the electroplating solutionwhen required. This enables the conversion coating to be formed in-situ,alongside the electrodeposition stage. The water stream (8) is requiredonly for a cold-water rinse to remove surplus solution, and a hot rinse.For maximum corrosion resistance the optional no-rinse sealing solutionis employed. The sealing solution provides a thin transparent film tothe coated article. The coated article is dried in a stream of air. Thewastewater stream (9) contains metal ions from the drag-in, and requiresno special treatment. The metal ions may be removed from the wastestream through the use of recovery techniques such as ion exchange. Thetreated water is then returned to the water stream (10). The metal ionscan be separated, or precipitated, and reused. The present invention canbe arranged such that it forms a closed loop. The electrolyte may alsobe passed through an external column containing activated carbon orsimilarly surface active materials to facilitate cleaning of theelectrolyte.

[0078]FIG. 3 illustrates a cell suitable for the operation of one aspectof the present invention. The article to be coated (12) is held on aconducting bar (13) and immersed in plating solution (14). An electricalheater (15) maintains the plating solution temperature at a constantvalue. An anode material (16) is present in the plating solution, andconnected to a power supply. Air is introduced into the cell through afrit (17), creating small bubbles and vigorous agitation of thesolution. The solutions comprising the conversion coating solution areheld in reservoirs above automatic titration devices (18). These possessvalves that release pre-set quantities of the solutions into the platingcell. The titrators are connected to ion selective electrodes (19) orother detection apparatus that determine the concentrations of thedifferent species in solution, and, if they fall below the requiredlimits, activate the titrators to release additional solution.

[0079]FIG. 4 shows the profile of the process according to one aspect ofthe present invention. This shows four consecutive treatment cycles. Incycle A, the voltage is increased sufficiently to permit theelectrodeposition of the substantially metallic coating. This isfollowed by a soak period (20) where the conversion coating is formed.Cycles B and C are identical. In Cycle D, additions of the conversioncoating formulation are made (21) in order to maintain theirconcentration in solution, since some of the solution would have beenconsumed in Cycles A, B and C.

EXAMPLE 1

[0080] According to the present invention, a solution comprises ceriumsulphate, Ce₂(SO₄)₃, 1 g/L, hydrogen peroxide, H₂O₂ (30 vol), 0.5 ml/L,and zinc sulphate heptahydrate, ZnSO₄.7H₂O, 220 g/L, in distilled water.

[0081] The solution was contained in a plastic vessel into which aplatinised titanium anode was placed. The plastic vessel was heated byan electrical element to a temperature of 25° C. The solution wasaspirated with air. A cleaned steel article was connected to thenegative terminal of a direct current power supply, whilst the anode wasconnected to the positive terminal. The steel article was immersed inthe solution thus forming the cathode in the cell, and current passedthrough the cell to provide a cathodic current density of 2 A/dm². Azinc coating was formed on the surface of the steel article that waslight grey in appearance, and after 20 mins, the thickness achieved was10 μm. The current was switched off and the conversion coating formationtook place, through reaction of the cerium salt and peroxide, forming anadherent and continuous layer on the coated article. After 2 mins thearticle was removed from the solution, rinsed in distilled water atambient temperature, rinsed in water at 60° C., and then dried in astream of air.

[0082] The conversion coating was light yellow with a brilliant lustre.The conversion coating was visually identical to traditional yellowchromate.

[0083] The adherence of the conversion coating was determined using atape test. Adhesive tape was pressed onto the surface of the conversioncoating and pulled off in one rapid movement. The tape was examined andthere were no signs of the conversion coating, indicating no loss ofadhesion.

EXAMPLE 2

[0084] The electroplating-conversion coating solution employed inExample 1 was reused after 30 mins. The sequence of treatment stagesdetailed in Example 1 was repeated. The conversion coating obtained wasidentical to that obtained in Example 1.

EXAMPLE 3

[0085] After a period of 24 hrs the solution employed in Example 2 wasreused. The sequence of treatment stages detailed in Example 1 wasrepeated, however, an additional aliquot of 0.5 ml/L 30 vol H₂O₂ wasprovided to the solution immediately following the electrodepositionstage. The conversion coating obtained was identical to that obtained inExample 1.

EXAMPLE 4

[0086] Solutions were prepared identical to those in Example 1, exceptthat H₂O₂ was excluded. The same sequence of treatment stages wasemployed, except that immediately following the electrodeposition stage,0.2 ml/L of H₂O₂ was added to the solution. The conversion coating wasformed on the surface of the zinc coating. After 5 mins, the coatedarticle was removed, and subject to rinsing identical to that inExample 1. The conversion coating obtained was identical to thatobtained in Example 1.

EXAMPLE 5

[0087] Solutions were prepared identical to that in Example 1, and thesame treatments used, except that following the electrodeposition stage,the current was reduced such that the cathode voltage increased to avalue of −0.8V, from −1.2V during electrodeposition. The conversioncoating was formed and after 4 mins, the current flow was switched off.The conversion coating thus obtained was identical to that obtained inExample 1.

EXAMPLE 6

[0088] According to the present invention, a solution comprises ceriumsulphate, Ce₂(SO₄)₃, 1 g/L, and hydrogen peroxide, H₂O₂, 30 vol, 1 ml/L,dissolved in distilled water. This was added to a solution comprisingzinc sulphate heptahydrate, ZnSO₄.7H₂O, 220 g/L and nickel sulphatehexahydrate, NiSO₄.6H₂O, 90 g/L.

[0089] The solution was contained in a plastic vessel into which aplatinised titanium anode was placed. The plastic vessel was heated byan electrical element to a temperature of 25° C. The solution wasaspirated with air. A cleaned steel article was connected to thenegative terminal of a direct current power supply, whilst the anode wasconnected to the positive terminal. The steel article was immersed inthe solution thus forming the cathode in the cell, and current passedthrough the cell to provide a cathodic current density of 2 A/dm². Azinc alloy coating, containing 10 weight per cent nickel, was formed onthe surface of the steel article that was light grey in appearance, andafter 20 mins, the thickness achieved was 10 μm. The current wasswitched off and the conversion coating formation took place, forming anadherent and continuous layer on the coated article. After 2 mins thearticle was removed from the solution, rinsed in distilled water atambient temperature, and then dried in a stream of air.

[0090] The conversion coating was light yellow with a brilliant lustre.The conversion coating was visually identical to traditional yellowchromate.

EXAMPLE 7

[0091] A solution and process identical to that described in Example 6was prepared and followed, except that cobalt chloride, CoCl₂, 50 g/L,was substituted for NiSO₄.6H₂O. The metallic coating obtained was a zincalloy containing 0.8 weight per cent cobalt. The conversion coating waslight yellow with a brilliant lustre. The conversion coating wasvisually identical to traditional yellow chromate.

EXAMPLE 8

[0092] A solution and process was employed identical to that inExample 1. Following the formation of the conversion coating and rinsestages, the coated article was immersed in a proprietary silane sealingsolution to which was added Ce₂(SO₄)₃, 1 g/L. The solution wasmaintained at 30° C. and the contact time was 2 mins.

[0093] The sealing solution provided a thin transparent film to thesurface of the yellow conversion coating.

[0094] The corrosion resistance of the sealed conversion coating wasdetermined through the used of a standard neutral salt spray test. Mildsteel panels treated using the method described in this example wereplaced in the test chamber with chromated zinc coatings of the samethickness. The time to the formation of white corrosion products, namelythe corrosion products of zinc, was estimated to determine the corrosionresistance of the conversion coatings. The time to the formation of redcorrosion products, namely the corrosion products of zinc, was estimatedto determine the corrosion resistance of the overall coating treatmentin its ability to protect steel.

[0095] The table provides the corrosion data obtained in hoursConversion Coating White rust Red rust Yellow chromate on Zinc 168 502Solution and Process of Example 8 168 670

[0096] The corrosion test demonstrates that the conversion coating ofthe present invention affords the same degree of corrosion protection tothe zinc coating as do traditional chromate finishes, and are moreprotective to the steel substrate.

EXAMPLE 9

[0097] According to the present invention, a solution comprises ceriumsulphate, Ce₂(SO₄)₃, 1 g/L, sodium silicate, Na₂SiO₄, 1 g/l, andhydrogen peroxide, H₂O₂, 30 vol, 1 ml/L, dissolved in distilled water.This was added to zinc sulphate heptahydrate, ZnSO₄.7H₂O, 220 g/L,

[0098] The solution was contained in a plastic vessel into which a steelanode was placed. The plastic vessel was heated by an electrical elementto a temperature of 25° C. The solution was aspirated with air. Acleaned steel article was connected to the negative terminal of a directcurrent power supply, whilst the anode was connected to the positiveterminal. The steel article was immersed in the solution thus formingthe cathode in the cell, and current passed through the cell to providea cathodic current density of 2A/dm². A zinc coating was formed on thesurface of the steel article that was light grey in appearance, andafter 20 mins, the thickness achieved was 10 μm. The current wasswitched off and the conversion coating formation took place, forming anadherent and continuous layer on the coated article. After 2 mins thearticle was removed from the solution, rinsed in distilled water atambient temperature, rinsed in water at 60° C., and then dried in astream of air.

EXAMPLE 10

[0099] According to the present invention, a solution comprises zincsulphate heptahydrate, ZnSO₄.7H₂O, 220 g/L, yttrium chloride, YCl₃, 1g/L, NdCl₃ 1 g/L and hydrogen peroxide, H₂O₂, 30 vol, 1.5 ml/L,dissolved in distilled water.

[0100] The solution was contained in a plastic vessel into which a steelanode was placed. The plastic vessel was heated by an electrical elementto a temperature of 25° C. The solution was aspirated with air. Acleaned steel article was connected to the negative terminal of a directcurrent power supply, whilst the anode was connected to the positiveterminal. The steel article was immersed in the solution thus formingthe cathode in the cell, and current passed through the cell to providea cathodic current density of 5A/dm². A zinc coating was formed on thesurface of the steel article that was light grey in appearance, andafter 20 mins, the thickness achieved was 10 μm. The current wasswitched off and the conversion coating formation took place, forming anadherent and continuous layer on the coated article. After 2 mins thearticle was removed from the solution, rinsed in distilled water atambient temperature, rinsed in water at 60° C., and then dried in astream of air.

[0101] The conversion coating was iridescent with a brilliant lustre.The conversion coating was visually identical to traditional iridescentchromate.

EXAMPLE 11

[0102] According to the present invention, a solution comprises zincsulphate heptahydrate, ZnSO₄.7H₂O, 220 g/L, praseodymium sulphate,Pr₂(SO₄)₃, 2 g/L, and hydrogen peroxide, H₂O₂, 30 vol, 0.3 ml/L,dissolved in distilled water.

[0103] The solution was contained in a plastic vessel into which aplatinised titanium anode was placed. The plastic vessel was heated byan electrical element to a temperature of 25° C. The solution wasaspirated with air. A cleaned steel article was connected to thenegative terminal of a direct current power supply, whilst the anode wasconnected to the positive terminal. The steel article was immersed inthe solution thus forming the cathode in the cell, and current passedthrough the cell to provide a cathodic current density of 2 A/dm². Azinc coating was formed on the surface of the steel article that waslight grey in appearance, and after 20 mins, the thickness achieved was10 μm. The current was switched off and the conversion coating formationtook place, forming an adherent and continuous layer on the coatedarticle. After 2 mins the article was removed from the solution, rinsedin distilled water at ambient temperature, rinsed in water at 60° C.,and then dried in a stream of air.

[0104] The conversion coating was light yellow with a brilliant lustre.The conversion coating was visually identical to traditional yellowchromate.

EXAMPLE 12

[0105] According to the present invention, a solution comprises zincsulphate heptahydrate, ZnSO₄.7H₂O, 220 g/L, cerium sulphate, Ce(SO₄)₂,2.5 g/L, lanthanum chloride, LaCl₃, 1 g/L, and hydrogen peroxide, H₂O₂,30 vol, 1 ml/L, dissolved in distilled water.

[0106] The solution was contained in a plastic vessel into which aplatinised anode was placed. The plastic vessel was heated by anelectrical element to a temperature of 25° C. The solution was aspiratedwith air. A cleaned steel article was connected to the negative terminalof a direct current power supply, whilst the anode was connected to thepositive terminal. The steel article was immersed in the solution thusforming the cathode in the cell, and current passed through the cell toprovide a cathodic current density of 2 A/dm². A zinc coating was formedon the surface of the steel article that was light grey in appearance,and after 20 mins, the thickness achieved was 10 μm. The current wasswitched off and the conversion coating formation took place, forming anadherent and continuous layer on the coated article. After 2 mins thearticle was removed from the solution, rinsed in distilled water atambient temperature, rinsed in water at 60° C., and then dried in astream of air.

[0107] The conversion coating was a golden yellow with a brilliantlustre.

EXAMPLE 13

[0108] According to the present invention, a solution comprises zincsulphate heptahydrate, ZnSO₄.7H₂O, 220 g/L, neodymium chloride, NdCl₃, 1g/L, and sodium nitrate, NaNO₃, 1 g/L, dissolved in distilled water.

[0109] The solution was contained in a plastic vessel into which aplatinised titanium anode was placed. The plastic vessel was heated byan electrical element to a temperature of 25° C. The solution wasaspirated with air. A cleaned steel article was connected to thenegative terminal of a direct current power supply, whilst the anode wasconnected to the positive terminal. The steel article was immersed inthe solution thus forming the cathode in the cell, and current passedthrough the cell to provide a cathodic current density of 2 A/dm². Azinc coating was formed on the surface of the steel article that waslight grey in appearance, and after 20 mins, the thickness achieved was10 μm. The current was switched off and the conversion coating formationtook place, forming an adherent and continuous layer on the coatedarticle. After 2 mins the article was removed from the solution, rinsedin distilled water at ambient temperature, rinsed in water at 60° C.,and then dried in a stream of air.

[0110] The conversion coating was light blue with a brilliant lustre.

1. A conversion coating solution comprises at least one precursor agent,at least one activator agent, and water solvent, wherein on provision toa solution suitable for the deposition of a substantially metalliccoating, provides an in-situ conversion coating to the surface of thesubstantially metallic coating.
 2. A solution according to claim 1,wherein the solution further comprises at least one modifying agent. 3.A solution according to claim 1 or claim 2, wherein the at least oneprecursor agent is a non-fully reducible metal ion. Preferably it ischosen from Y³⁺, Rb⁺, Li⁺, La³⁺, Ce³⁺, Ce⁴⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺,Gd³⁺, Td³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺.
 4. A solution accordingto claim 3 wherein the at least one precursor agent is chosen from Y³⁺,La³⁺, Ce³⁺, Ce⁴⁺, Pr³⁺, Nd³⁺.
 5. A solution according to claim 1 orclaim 2, wherein the at least one activator agent comprises a peroxide,peroxonium salt, a hydroperoxide, ozone analogue of peroxide, ornitrate.
 6. A solution according to claim 1 or claim 2, wherein theactivator agent is peroxide.
 7. A solution according to claim 1 or claim2, wherein the solution suitable for the deposition of a substantiallymetallic coating comprises at least one of the following: Zn²⁺, Fe²⁺,Fe³⁺, Ni²⁺, Co²⁺, Ag⁺, Sn²⁺, Sn³⁺, Cu⁺, Cu²⁺, Mn²⁺, Mn³⁺, Mn⁴⁺, Mn⁶⁺. 8.A solution according to claim 7, wherein the solution suitable for thedeposition of a substantially metallic coating comprises at least one ofthe following: Zn²⁺, Fe²⁺, Fe³⁺, Ni²⁺, C²⁺.
 9. A solution according toclaim 7, wherein the solution suitable for the electrodeposition of asubstantially metallic coating is an electroplating solution.
 10. Asolution according to claim 7, wherein the solution suitable for theelectrodeposition of a substantially metallic coating is a proprietaryelectroplating solution.
 11. A solution according to any preceding claimfurther comprises a conversion coating modifying agent, comprising anon-metallic anion.
 12. A solution according to claim 11, wherein thenon-metallic anion comprises one of silicate, oxalate, iodate, formate,phosphate, or carbonate.
 13. A solution according to any precedingclaim, wherein the solvent is acid or alkaline.
 14. A solution accordingto any preceding claim, wherein the solvent is water.
 15. An articletreated using a solution according to any previous claim.
 16. A processfor coating an article, wherein a conversion coating solution comprisesat least one precursor agent, at least one activator agent, and watersolvent, wherein on provision to a solution suitable for the depositionof substantially metallic coatings, imparts a conversion coating layerto the substantially metallic coating.
 17. A process for coating anarticle wherein a conversion coating solution comprises at least oneprecursor agent, and water solvent, wherein use is provided to anaqueous solution suitable for the deposition of substantially metalliccoatings, wherein an article is brought into contact with the solution,wherein use a substantially metallic coating is formed on the article,the at least one activator agent is provided to the solution, such thata conversion coating layer is imparted to the substantially metalliccoating.
 18. A process for coating an article wherein a conversioncoating solution comprises at least one precursor agent, and watersolvent, wherein use is provided to an aqueous solution suitable for theelectrodeposition of substantially metallic coatings, an article isbrought into contact with the solution, electrical current is passed tothe article such that a substantially metallic coating is deposited ontothe article, the current is substantially reduced or halted, the atleast one activator agent is provided to the solution, such that aconversion coating layer is imparted to the substantially metalliccoating.
 19. A process for coating an article wherein a conversioncoating solution comprises at least one activator agent, and watersolvent, wherein use is provided to an aqueous solution suitable for theelectrodeposition of substantially metallic coatings, an article isbrought into contact with the solution, electrical current is passed tothe article such that a substantially metallic coating is deposited ontothe article, the current is substantially reduced or halted, the atleast one precursor agent is provided to the solution, such that aconversion coating layer is imparted to the substantially metalliccoating.
 20. A process according to any of claims 15 to 19, wherein theactivator agent comprises a peroxide or nitrate.
 21. A process accordingto any of claims 15 to 20, wherein the precursor agent comprises Y³⁺,La³⁺, Ce³⁺, Ce⁴⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺,Tm³⁺, Yb³⁺, Lu³⁺.
 22. A process according to claim 21 wherein the atleast one precursor agent is chosen from Y³⁺, La³⁺, Ce³⁺, Ce⁴⁺, Pr³⁺,Nd³⁺.
 23. A process according to any of claims 15 to 22, wherein thesolution suitable for the deposition of a substantially metallic coatingcomprises at least one of the following: Zn²⁺, Fe²⁺, Fe³⁺, Ni²⁺, Co²⁺,Ag⁺, Sn²⁺, Sn³⁺, Cu⁺, Cu²⁺, Mn²⁺, Mn³⁺, Mn⁴⁺, Mn⁶⁺.
 24. A processaccording to claim 23, wherein the suitable for the deposition of asubstantially metallic coating comprises at least one of the following:Zn²⁺, Fe²⁺, Fe³⁺, Ni²⁺, Co²⁺.
 25. A process according to any of claims15 to 24, wherein the solution suitable for the electrodeposition of asubstantially metallic coating is an electroplating solution.
 26. Aprocess according to claim 25, wherein the solution suitable for theelectrodeposition of a substantially metallic coating is a proprietaryelectroplating solution.
 27. A process according to any of claims 15 to26, wherein the solution further comprises a modifying agent.
 28. Aprocess according to claim 27, wherein the modifying agent is anon-metallic anion comprising at least one of silicate, oxalate, iodate,formate, phosphate, or carbonate.
 29. A process according to any ofclaims 15 to 28, wherein the solvent in acid or alkaline.
 30. A processaccording to any of claims 15 to 29, wherein the solvent is water.
 31. Aprocess according to any of claims 15 to 30, wherein the activator isprovided to the solution prior to electrodeposition.
 32. A processaccording to any of claims 15 to 30, wherein the activator is providedto the solution during electrodeposition.
 33. A process according to anyof claims 15 to 30, wherein the activator is provided to the solutionfollowing electrodeposition.
 34. A process according to any of claims 15to 33, wherein the electrical current provided to reduce metal ions inthe plating bath to their metallic state is direct current.
 35. Aprocess according to any of the claims 15 to 34, wherein the electricalcurrent provided to reduce metal ions in the plating bath to theirmetallic state is pulsed current.
 36. A process according to any claims15 to 35, wherein the electrical current provided to the plating bath isdirect current superimposed with alternating current.
 37. A processaccording to any claims 15 to 36, wherein the substrate is a metal. 38.A process according to claim 37, wherein the substrate is steel.
 39. Aprocess according to any of claims 15 to 38, where the substrate is anon-metal.
 40. A process according to claim 39, wherein the substrate isa plastic or carbon fibre.
 41. An article treated using a processaccording to any of claims 15 to 40.